Unless otherwise indicated herein, the materials described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
An inductive motor converts electrical energy to mechanical energy via electromagnetic interactions that create torque on a shaft. An AC motor may include a stator in which a magnetically polarized rotor rotates. The stator can include a structure on which a conductive winding is wound in a configuration such that a rotating magnetic field is created within the stator when AC current flows through the winding. The rotor may include one or more permanent magnets or may be configured to become magnetized via inductive interaction with the stator's magnetic field (e.g., via conductive coils and/or ferromagnetic materials in the rotor). When an AC source is applied to the winding, the stator's magnetic field can cause the rotor to rotate relative to the stator. The rotor can be coupled to a shaft, which transfers the torque applied to the rotor, and the mechanical energy can then be used to perform work on a load. The rate at which work can be performed using the motor (i.e., the output power of the motor) is related to the torque magnetically applied to the rotor. The torque is proportionate to the strength of the magnetic field imparted on the rotor by the stator's winding. And the strength of the magnetic field is proportionate to the current through the winding, and the number of turns in the winding. The number of turns in the winding is a feature of the winding's geometry, and the current depends on the resistivity of the wire used and also the voltage of the AC source.
Typical AC inductive motors used in compressors, fans, and a variety of other household appliances and electronics are designed to operate under maximum anticipated load conditions, and to do so even when supplied with less than a nominally expected AC driving voltage. Providing for a tolerance in the AC driving voltage helps ensure reliable operation of the motor when supplied with a line voltage that is less from the nominally expected value. For example, a motor may be designed to drive a maximum anticipated load when supplied with a line voltage of about 105 VAC, rather than 120 VAC.
To provide such a tolerance in AC driving voltage, a motor designer first determines the magnetic field strength necessary to provide a target output power level. The motor designer then selects a stator with a conductive winding that will conduct a sufficient amount of current to provide the necessary magnetic field strength. As noted above, the magnetic field strength is proportionate to the current through the stator's winding, which is itself proportionate to the AC driving voltage. Thus, to account for the voltage tolerance, the conductive winding is formed of wire with a resistivity sufficient to conduct a desired amount of current (sufficient to provide the necessary magnetic field strength) while driven with the reduced AC voltage rather than the nominally expected voltage. As such, a typical motor's windings are over-sized for operation at the nominal AC voltage, and generally conduct more current than actually necessary, which wastes energy. For example, the stator winding may use a lower gauge wire than necessary to generate the desired magnetic field strength when supplied with the nominally expected AC driving voltage.
The efficiency of an AC motor can be expressed by a ratio of the electrical power consumed (e.g., the product of voltage and current supplied to the motor from the AC source) and the actual power delivered to a load to perform work. The ratio of consumed power to volt-amperes of delivered power is referred to as a motor power factor. For typical motors, the power factor varies depending on the power consumed, but also varies depending on the load being driven. In most cases, a motor operates with its greatest power factor when under maximum loaded conditions, because a relatively small fraction of the consumed power is dissipated as eddy currents or resistive heating. By contrast, when the motor is not under maximum load, the motor consumes relatively more excess energy. The excess energy is dissipated as heat (e.g., due to eddy currents in the stator structure or resistive heating in the coils). Essentially, when a motor is only partially loaded, the excess energy that is not used to perform work is simply wasted.
For example, a typical motor rated for ⅓ horsepower may draw about 5 amperes from a 120 VAC source when fully loaded and operate around 3,500 revolutions per minute (RPM) with about 70-80% power factor or better. Yet, even under virtually no load, the same motor may continue to draw about 3.8 amperes, although with a reduced power factor of about 20-30%. It is under the unloaded condition (or under a state of reduced load) where considerable excess electric power is wasted because of the decreased efficiency of the motor under such conditions. To a large extent, the decreased efficiency is caused by the eddy-current and resistive losses noted above.
One way to decrease the amount of power dissipated in the motor is to decrease the amount of power supplied to the motor from the AC source. A reduced AC voltage drives a reduced current through the stator coils, which reduces resistive losses in the coils themselves and also provides a reduction of the magnetic field strength in the stator which results in less eddy-current loss as well. Reducing the supplied AC voltage can cause an unloaded (or partially loaded) motor to draw less current, which reduces the electrical energy consumed, and thereby increases the efficiency of the motor. However, regulating an AC voltage provided to a motor typically involves switching losses and is energy inefficient itself.